m68knommu: disable MC68000 cpu target when MMU is selected

As pointed out by Geert, MC68000 target needs to be disabled when
MMU support is enabled.

From Geert:

This needs a "depends on !MMU".
Else allmodconfig will select it, causing -m68000 to be passed to the assembler,
which may break the build depending on your version of binutils, a.o.

arch/m68k/kernel/entry.S:186: Error: invalid instruction for this
architecture; needs 68020 or higher (68020 [68k, 68ec020], 68030
[68ec030], 68040 [68ec040], 68060 [68ec060]) -- statement `bfextu
%sp@(50){#0,#4},%d0' ignored
arch/m68k/kernel/entry.S:211: Error: invalid operand mode for this
architecture; needs 68020 or higher -- statement `jbsr
@(sys_call_table,%d0:l:4)@(0)' ignored

Cfr. http://kisskb.ellerman.id.au/kisskb/buildresult/7416877/



Signed-off-by: Luis Alves <ljalvs@gmail.com>
Signed-off-by: Greg Ungerer <gerg@uclinux.org>